BACKGROUND OF THE INVENTION
[0001] The present invention relates to a method for refining inorganic short fibers such
as carbon nanotubes (CNTs).
[0002] The CNTs generated by an arc discharge method and a laser evaporation method generally
include impurities, which are not CNT. For example, such CNTs include carbonaceous
matter, such as graphites and fullerenes. In a case where metal catalysts are used
to generate the CNTs, the CNTs further include the metal catalysts as impurities.
[0003] Japanese Laid-Open Patent Publication No. 8-198611 discloses a method for removing impurities included in CNTs. In the method of the
publication, a mixture of CNTs and impurities goes through a centrifugal separation
or a floatation to remove carbonaceous matter other than the CNTs. If CNTs include
metal catalysts, acid is added in the mixture of the CNTs and the impurities to dissolve
and remove the metal catalysts. Alternatively, the mixture of CNTs and impurities
is passed through a magnetic field. However, in the above mentioned methods, CNTs
and impurities are not reliably separated. Therefore, CNTs are not refined with a
high yield rate.
[0004] The
Japanese patent application JP 2000 072422 discloses a method and an apparatus for sorting and recovering carbon nanotubes(CNT's)
from a carbon material mixture obtained by an arc discharge method, etc.. A DC electrophoresis
refining apparatus 26 consists of a main body vessel 6 which is filled with a migration
liquid and induces electrophoresis, electrodes which are arranged on the opposite
surfaces of this vessel 6, a DC power source 8 which impresses DC to one of these
electrodes to an anode 10 and the other to a cathode 12, a dispersion migration liquid
supplying means 30 which supplies the dispersion migration liquid dispersed with the
carbon material mixture into the main body vessel 6 and fluidizes the dispersion migration
liquid in one direction between the electrodes and a recovering means 38 which is
disposed in the position where the carbon nanotubes gather in the outlet side of the
dispersion migration liquid. The apparatus is so constituted that the carbon material
mixture containing the carbon nanotubes(CNT's) are separated and coordinated by every
kind in the inter-electrode direction by the electrophoresis and that the carbon nanotubes
are sorted and recovered by the recovering means.
SUMMARY OF THE INVENTION
[0005] Accordingly, it is an objective of the present invention to provide a method for
refining inorganic short fibers at a high yield rate.
[0006] To achieve the above objective, the present invention provides a method for refining
inorganic short fibers. The method according to claim 1 includes:
dispersing unrefined carbon nanotubes, which contain impurities including nonfibrous
carbons and metal catalysts, into a dielectric liquid containing a surface active
agent;
causing carbon nanotubes to precipitate out of the dielectric liquid separate from
the nonfibrous carbons and metal catalysts based solely on the differences between
the rates of sedimentation of the carbon nanotubes, nonfibrous carbon, and metal catalysts
by applying an electric field having an intensity of 1 to 15 kV/cm to the dielectric
liquid to cause the carbon nanotubes in the dielectric liquid to electrostatically
bond end to end with one another in a direction parallel to the direction of the electric
field such that the size of the carbon nanotubes bonded together is greater than the
size of the nonfibrous carbons in the dielectric liquid and the bonded carbon nanotubes
fall through the dielectric liquid at a rate that is faster than the rate at which
the nonfibrous carbons fall through the dielectric liquid but slower than the rate
at which the metal catalysts fall through the dielectric liquid so as to form a three-layer
sediment having a bottom layer of metal catalysts, an intermediate layer of carbon
nanotubes formed above the layer of metal catalysts, and a top layer of nonfibrous
carbons formed above the layers of metal catalysts and carbon nanotubes;
removing, after formation of the three-layer sediment, the dielectric liquid and the
layer of nonfibrous carbons; and
collecting the precipitated carbon nanotubes after the dielectric liquid and the layer
of nonfibrous carbons have been removed.
[0007] Further advantageous developments are set forth in the accompanied dependent claims
[0008] Other aspects and advantages of the invention will become apparent from the following
description, taken in conjunction with the accompanying drawings, illustrating by
way of example the principles of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] The invention, together with objects and advantages thereof, may best be understood
by reference to the following description of the presently preferred embodiments together
with the accompanying drawing in which:
Fig. 1 is a schematic view illustrating an electrostatic orientation apparatus used
for refining inorganic short fibers according to a preferred embodiment.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0010] A preferred embodiment of the present invention will now be described with reference
to Fig. 1.
[0011] In a method for refining inorganic short fibers according to the preferred embodiment,
an electrostatic orientation apparatus 10 shown in Fig. 1 is used. The electrostatic
orientation apparatus 10 includes a container 12. The container 12 has a pair of inner
side surfaces, which face each other. First and second electrodes 14 and 15 are each
attached to one of the inner side surfaces and are connected to a voltage applying
apparatus 13. The voltage applying apparatus 13 applies an alternating high voltage
to the first and second electrodes 14, 15.
[0012] A drain pipe 17 is located at the bottom of the container 12. A faucet 18 is attached
to the drain pipe 17. A filter 17a is attached to the inner bottom surface of the
container 12 and covers the upper opening of the drain pipe 17.
[0013] The container 12 stores dielectric liquid 20. The dielectric liquid 20 is liquid
that is polarized when an electric field is applied. Concrete examples of the dielectric
liquid 20 include silicone oil, halogenated hydrocarbon, such as carbon tetrachloride,
n-hexane, and cyclohexane. The silicone oil is preferably used after reducing the
viscosity.
[0014] A method for refining inorganic short fibers according to the preferred embodiment
will now be described. An example of a method for refining inorganic short fibers,
which is CNTs in the preferred embodiment, is discussed below. The CNTs to be refined
may be either single-wall carbon nanotubes or multi-wall carbon nanotubes.
[0015] The dielectric liquid 20 is filled in the container 12 of the electrostatic orientation
apparatus 10 in advance. Unrefined CNTs are added to and dispersed in another dielectric
liquid 20 separate from the dielectric liquid 20 that is in the container 12. The
unrefined CNT is generated by, for example, an arc discharge method and includes first
and second impurities. In this embodiment, the first impurities are metal catalysts,
and the second impurities are nonfibrous carbons, such as graphites and fullerenes.
The dielectric liquid 20 preferably includes an appropriate amount of surface active
agent. The surface active agent causes the CNTs to be reliably dispersed in the dielectric
liquid 20. A preferable surface active agent is a nonionic surface active agent.
[0016] The voltage applying apparatus 13 applies an alternating high voltage to the first
and second electrodes 14, 15. In this state, the dielectric liquid 20 in which the
unrefined CNTs are dispersed is supplied to the container 12 using a supplying container
19. The supplied dielectric liquid 20 is then mixed with the dielectric liquid 20
in the container 12. As a result, an electric field formed between the first electrode
14 and the second electrode 15 is applied to the dielectric liquid 20 and the CNTs.
The electric field applied to the dielectric liquid 20 and the CNTs preferably has
an intensity of 1 to 15kV/cm. The unrefined CNT content of the dielectric liquid 20
in the container 12 is preferably approximately 500mg per 100ml of the dielectric
liquid 20.
[0017] When the electric field is applied to the CNTs in the dielectric liquid 20, one end
of each CNT is oriented toward the first electrode 14 and the other end of the CNT
is oriented toward the second electrode 15. In other wards, the CNTs in the dielectric
liquid 20 to which an electric field is applied are aligned along the electric field
direction. The CNTs in the dielectric liquid 20 are also polarized when the electric
field is applied. Each end of the polarized CNT is electrically combined with one
of the ends of another polarized CNT. In other words, the polarized CNTs are bonded
with each other.
[0018] When the dielectric liquid 20 that includes the unrefined CNTs is left at rest while
the electric field is applied, the unrefined CNTs start to precipitate. The dimension
of the bonded CNTs is greater than the dimension of the nonfibrous carbon. Therefore,
the falling rate (sedimentation rate) of the bonded CNTs is greater than the falling
rate of the nonfibrous carbon. The gravity of the metal catalysts is greater than
the gravity of the CNTs and the nonfibrous carbons. Therefore, the falling rate of
the metal catalysts is greater than the falling rate of the bonded CNTs and the nonfibrous
carbon. Thus, sediment includes a layer of the metal catalysts, a layer of the CNTs,
which is formed above the metal catalysts layer, and a layer of the nonfibrous carbon,
which is formed above the CNTs layer.
[0019] The faucet 18 is opened after sediment is sufficiently stored at the bottom of the
container 12. Accordingly, the dielectric liquid 20 in the container 12 is drained
through the drain pipe 17. After that, the nonfibrous carbons are removed from the
top of the sediment remained in the container 12 and the CNTs are collected subsequently.
The CNTs are refined as described above.
[0020] The preferred embodiment provides the following advantages.
[0021] The falling rate of the bonded CNTs greatly differs from the falling rate of the
impurities. Therefore, the CNTs and the impurities are reliably separated by the refining
method of the preferred embodiment. Accordingly, the CNTs are refined at a high yield
rate.
[0022] After the dielectric liquid 20 is drained through the drain pipe 17, the precipitated
CNTs are collected from the upper part of the sediment. This prevents the impurities
from being mixed with the CNTs when collecting the CNTs. The precipitated CNTs can
also be drained through the drain pipe 17 with the dielectric liquid 20 to be collected.
However, in this case, the possibility that the precipitated impurities are mixed
with the CNTs increases.
[0023] When the electric field applied to the CNTs is formed based on a direct voltage,
the electric charge is transferred between the bonded CNTs. As a result, the bond
of the CNTs might be dissociated. However, in the refining method of the preferred
embodiment, the electric field applied to the CNTs is formed based on the alternating
voltage. In this case, the direction of the electric field is reversed before the
electric charge is transferred between the bonded CNTs. This prevents the dissociation
of the bond of the CNTs.
[0024] The dielectric liquid 20 in which the unrefined CNTs is dispersed is supplied to
the container 12 at once. Therefore, the CNTs are separated from the impurities without
agitating the dielectric liquid 20 in the container 12 and are precipitated out of
the dielectric liquid 20. If the dielectric liquid 20 in which the unrefined CNTs
are dispersed is supplied to the container 12 sequentially, the CNTs in the dielectric
liquid 20 supplied earlier might precipitate at the same time as the metal catalysts
in the dielectric liquid 20 supplied afterward. To prevent this, the dielectric liquid
20 in the container 12 needs to be agitated while supplying the dielectric liquid
20 in which the unrefined CNTs are dispersed.
[0025] It should be apparent to those skilled in the art that the present invention may
be embodied in many other specific forms without departing from the spirit or scope
of the invention. Particularly, it should be understood that the invention may be
embodied in the following forms.
[0026] The intensity of the electric field applied to the dielectric liquid 20 and the CNTs
in the container 12 need not be 1 to 15kV/cm. However, if the intensity of the applied
electric field is less than 1kV/cm, the CNTs can be insufficiently oriented in a desired
direction. If the intensity of the applied electric field is greater than 15kV/cm,
the dielectric liquid 20 in the container 12 is agitated, which hinders the CNTs from
being oriented in a desired direction.
[0027] Amount of the unrefined CNTs added to the dielectric liquid 20 need not be 500mg
per 100ml of the dielectric liquid 20.
[0028] The filter 17a may be omitted. In this case, the faucet 18 is opened when the metal
catalysts have precipitated such that the metal catalysts are drained through the
drain pipe 17 with the dielectric liquid 20. At the time the metal catalysts have
precipitated, the nonfibrous carbons float at the upper portion of the dielectric
liquid 20 and the CNTs float at the lower portion of the dielectric liquid 20. Therefore,
the CNTs are separated from the nonfibrous carbons by closing the faucet 18 after
draining the metal catalysts and collecting the upper portion of the dielectric liquid
20. When the remaining dielectric liquid 20 is left at rest, the CNTs precipitate.
When the faucet 18 is opened again, the precipitated CNTs are drained through the
drain pipe 17 with the dielectric liquid 20. In this modified embodiment, the CNTs
and the impurities are separated before the nonfibrous carbons precipitate. This reduces
the time required for refining.
[0029] The present invention may be applied to refining of carbon short fibers other than
CNTs, such as carbon nanohorn and carbon microcoil. The carbon microcoil generated
by a chemical vapor deposition (CVD) method has a high purity as compared to the CNTs
generated by the arc discharge method but includes metal catalysts as impurities.
The carbon microcoil may either have two adjacent loops connected to each other or
the loops separate from each other.
[0030] The present invention may be applied to refining of boron nitride nanotube. The present
invention may also be applied to refining of nanotube formed of silicon carbide (SiC),
silicon oxide (SiO
2), aluminum oxide (Al
2O
3), vanadium oxide (V
2O
5), molybdenum oxide (MoO
3), and titanium oxide (TiO
2). In the case where the present invention is applied to refining of inorganic short
fibers other than the CNTs, the intensity of the electric field applied to the inorganic
short fibers and the density of the inorganic short fibers in the dielectric liquid
20 may be changed as required.
[0031] The electric field applied to the CNTs need not be formed based on the alternating
voltage but may be formed based on the direct voltage. In the case the electric field
applied to the CNTs is formed based on the direct voltage, the dissociation of the
bond of the CNTs is suppressed by applying conductivity on the surface of the CNTs
or by changing the dielectric liquid or the surface active agent.
[0032] Therefore, the present examples and embodiments are to be considered as illustrative
and not restrictive and the invention is not to be limited to the details given herein,
but may be modified within the scope and equivalence of the appended claims.
[0033] A method for refining inorganic short fibers at a high yield rate. In the refining
method of inorganic short fibers according to the present invention, an electric field
is applied to dielectric liquid in which inorganic short fibers that include impurities
are dispersed. The inorganic short fibers to which the electric field is applied are
polarized and bonded with each other. The inorganic short fibers in the dielectric
liquid are caused to fall. The falling inorganic short fibers are collected separately
from the falling impurities utilizing the difference between the falling rate of the
bonded inorganic fibers and the falling rate of the impurities.
1. A method for refining carbon nanotubes, the method being
characterized by:
dispersing unrefined carbon nanotubes, which contain impurities including nonfibrous
carbons and metal catalysts, into a dielectric liquid containing a surface active
agent;
causing carbon nanotubes to precipitate out of the dielectric liquid separate from
the nonfibrous carbons and metal catalysts based solely on the differences between
the rates of sedimentation of the carbon nanotubes, nonfibrous carbon, and metal catalysts
by applying an electric field having an intensity of 1 to 15 kV/cm to the dielectric
liquid to cause the carbon nanotubes in the dielectric liquid to electrostatically
bond end to end with one another in a direction parallel to the direction of the electric
field such that the size of the carbon nanotubes bonded together is greater than the
size of the nonfibrous carbons in the dielectric liquid and the bonded carbon nanotubes
fall through the dielectric liquid at a rate that is faster than the rate at which
the nonfibrous carbons fall through the dielectric liquid but slower than the rate
at which the metal catalysts fall through the dielectric liquid so as to form a three-layer
sediment having a bottom layer of metal catalysts, an intermediate layer of carbon
nanotubes formed above the layer of metal catalysts, and a top layer of nonfibrous
carbons formed above the layers of metal catalysts and carbon nanotubes;
removing, after formation of the three-layer sediment, the dielectric liquid and the
layer of nonfibrous carbons; and
collecting the precipitated carbon nanotubes after the dielectric liquid and the layer
of nonfibrous carbons have been removed.
2. The method according to claim 1, characterized in that the applied electric field is formed based on an alternating voltage.
3. The method according to claim 1 or 2, characterized in that the unrefined carbon nanotubes are dispersed into the dielectric liquid such that
approximately 500 mg of the unrefined carbon nanotubes are contained per 100 ml of
the dielectric liquid.
4. The method according to any one of claims 1 to 3, characterized in that the surface active agent is a nonionic surface active agent.
1. Verfahren zum Raffinieren von Kohlenstoffnanoröhren, wobei das Verfahren
gekennzeichnet ist durch:
Dispergieren unraffinierter Kohlenstoffnanoröhren, welche Verunreinigungen enthalten,
die nicht-fasrige Kohlenstoffe und Metallkatalysatoren beinhalten, in einer ein grenzflächenaktives
Mittel enthaltenden dielektrischen Flüssigkeit;
Veranlassen, dass Kohlenstoffnanoröhren getrennt von den nicht-fasrigen Kohlenstoffen
und Metallkatalysatoren aus der dielektrischen Flüssigkeit ausfallen, nur basierend
auf den Unterschieden zwischen den Sedimentationsgeschwindigkeiten der Kohlenstoffnanoröhren,
des nicht-fasrigen Kohlenstoffs und der Metallkatalysatoren, durch Anlegen eines elektrischen Feldes mit einer Intensität von 1 bis 15 kV/cm an die
dielektrische Flüssigkeit, um zu veranlassen, dass sich die Kohlenstoffnanoröhren
in der dielektrischen Flüssigkeit elektrostatisch Ende an Ende in einer Richtung parallel
zu der Richtung des elektrischen Feldes aneinander binden, sodass die Größe der aneinander
gebundenen Kohlenstoffnanoröhren größer ist als die Größe der nicht-fasrigen Kohlenstoffe
in der dielektrischen Flüssigkeit, und die gebundenen Kohlenstoffnanoröhren durch die dielektrische Flüssigkeit mit einer Geschwindigkeit absinken, die schneller ist
als die Geschwindigkeit, mit welcher die nicht-fasrigen Kohlenstoffe durch die dielektrische Flüssigkeit absinken, aber langsamer ist als die Geschwindigkeit,
mit welcher die Metallkatalysatoren durch die dielektrische Flüssigkeit absinken, um ein Drei-Schicht-Sediment zu bilden, das
eine Unterschicht aus Metallkatalysatoren, eine über der Schicht aus Metallkatalysatoren
gebildete Zwischenschicht aus Kohlenstoffnanoröhren, und eine über den Schichten aus
Metallkatalysatoren und Kohlenstoffröhren gebildete obere Schicht aus nicht-fasrigen
Kohlenstoffen ausweist;
Entfernen der dielektrischen Flüssigkeit und der Schicht aus nicht-fasrigen Kohlenstoffen
nach der Bildung des Drei-Schicht-Sediments; und
Sammeln der ausgefällten Kohlenstoffnanoröhren, nachdem die dielektrische Flüssigkeit
und die Schicht aus nicht-fasrigen Kohlenstoffen entfernt wurden.
2. Verfahren nach Anspruch 1, dadurch gekennzeichnet, dass das angelegte elektrische Feld basierend auf einer Wechselspannung erzeugt wird.
3. Verfahren nach Anspruch 1 oder 2, dadurch gekennzeichnet, dass die unraffinierten Kohlenstoffnanoröhren in der dielektrischen Flüssigkeit so dispergiert
werden, dass ungefähr 500 mg der unraffinierten Kohlenstoffnanoröhren pro 100 mL der
dielektrischen Flüssigkeit enthalten sind.
4. Verfahren nach einem der Ansprüche 1 bis 3, dadurch gekennzeichnet, dass das grenzflächenaktive Mittel ein nichtionisches grenzflächenaktives Mittel ist.
1. Procédé pour raffiner des nanotubes de carbone, le procédé étant
caractérisé par le fait de:
disperser des nanotubes de carbone non raffinés, qui contiennent des impuretés incluant
des carbones non fibreux et des catalyseurs métalliques, dans un liquide diélectrique
contenant un agent tensioactif;
amener des nanotubes de carbone à former un précipité par rapport au liquide diélectrique
en se séparant des carbones non fibreux et des catalyseurs métalliques uniquement
suivant les différences entre les vitesses de sédimentation des nanotubes de carbone,
carbones non-fibreux, et catalyseurs métalliques en appliquant un champ électrique
ayant une intensité de 1 à 15 kV/cm au liquide diélectrique pour amener les nanotubes
de carbone dans le liquide diélectrique à se lier les uns aux autres bout à bout de
manière électrostatique dans une direction parallèle à la direction du champ électrique
de façon à ce que la taille des nanotubes de carbone liés soit plus grande que la
taille des carbones non-fibreux dans le liquide diélectrique et à ce que les nanotubes
de carbone liés se précipitent à travers le liquide diélectrique à une vitesse qui
est plus rapide que la vitesse à laquelle les carbones non-fibreux se précipitent
à travers le liquide diélectrique mais plus lente que la vitesse à laquelle les catalyseurs
métalliques se précipitent à travers le liquide diélectrique de manière à former un
sédiment à trois couches ayant une couche inférieure de catalyseurs métalliques, une
couche intermédiaire de nanotubes de carbone formée au-dessus de la couche de catalyseurs
métalliques, et une couche supérieure de carbones non-fibreux formée au-dessus des
couches de catalyseurs métalliques et nanotubes de carbone;
retirer, après formation du sédiment à trois couches, le liquide diélectrique et la
couche de carbones non-fibreux; et
collecter les nanotubes de carbone précipités après retrait du liquide diélectrique
et de la couche de carbones non-fibreux.
2. Procédé selon la revendication 1, caractérisé en ce que le champ électrique appliqué est formé sur la base d'une tension alternative.
3. Procédé selon la revendication 1 ou 2, caractérisé en ce que les nanotubes de carbone non-raffinés sont dispersés dans le liquide diélectrique
de façon à ce que 100 ml du liquide diélectrique contiennent environ 500 mg de nanotubes
de carbone non raffinés.
4. Procédé selon l'une quelconque des revendications 1 à 3, caractérisé en ce que l'agent tensioactif est un agent tensioactif non-ionique.